1. Field of the Invention
This invention relates to a method and apparatus for automatically measuring the viscosity of a solution or other liquid.
2. Description of the Prior Art
Many methods and apparatus for measuring viscosities of liquids are known and described in the prior art. One method of measuring the viscosity of a liquid is to cause it to flow along a fine transparent tube, usually a capillary tube, and to note the time taken for the liquid to pass between two spaced points on the tube. The viscosity can be determined from the time interval so obtained by comparing the time of flow with the time taken by a standard fluid, such as water or aniline, having a viscosity that is already known in identical conditions of temperature, pressure and so forth. This is a tedious procedure and liable to error.
Automatic viscosity measuring devices such as disclosed in U.S. Pat. No. 3,071,961 are known to the art. In this reference, paired light beam photodetectors are used to detect the downward flow of oil under controlled conditions from which the viscosity can be computed.
U.S. Pat. No. 3,713,328 discloses apparatus for automatically measuring the viscosity of a liquid comprising a Lantz-Zeitfuchs type reverse flow viscometer, a timing means actuated by photoelectric devices consisting of pairs of photoelectric cell and light source, and a sequence control system. The viscometer is placed in a constant temperature bath and has a timing bulb encasing light source lamps and photoelectric cells in pairs at the upper and lower timing marks of the bulb.
Various other types of sensors are known for measuring viscosities of liquids by detecting the time interval of passage of the liquid between two points in a capillary tube. The use of spark detectors to detect a meniscus of a flowing oil is known and can be used in place of photodetectors. A number of disadvantages are associated with the use of either photocell detectors or spark detectors. Photocell detectors are not satisfactory for use in measuring black oils and they require an inordinate amount of space and wiring to make them operational. On the other hand a spark detector system requires a high voltage circuit that may cause difficulties with other circuits in the instrument. In addition, the spark system generates excessive electronic noise, particularly in the low voltage control circuits and in the many computer circuits and there is a potential explosion hazard when high-voltage sparks are used. Water in the oil can interfere with spark detectors.
U.S. Pat. No. 3,798,960 discloses the use of thermistors mounted in a capillary wall and extending into the flowing stream to detect the passage of a meniscus.
U.S. Pat. No. 3,939,406 discloses use of microwaves to detect pressure drop and velocity of fluid flow. Therein, a microwave fluid flow meter is described utilizing two spaced microwave sensors positioned along a fluid flow path. Each sensor includes a microwave cavity having a frequency of resonance dependent upon the static pressure of the fluid at the sensor locations. The resonant response of each cavity with respect to a variation in pressure of the monitored fluid is represented by a corresponding electrical output which can be calibrated into a direct pressure reading. The pressure drop between sensor locations is then correlated as a measure of fluid velocity.
A radiation source has been known to be useful in detecting meniscus positions in capillary tube type automatic viscometers. U.S. Pat. No. 3,908,441 discloses a liquid level detection device which includes a transparent tube for containing a liquid which forms a meniscus in the tube. A radiation source is placed facing the periphery of the tube and a photoelectric device faces the periphery of the same part of the tube but is angularly offset with respect to the light from the source incident on the tube, and receives light reflected from the internal surface of the tube when no liquid is present.
Ultrasonic transducers are known to be useful in detecting the level of liquid contained in vessels or vats.
Finally, ultrasonic emitters and detectors have been used to measure liquid viscosity, the presence and orientation of particles in a mixture and certain other properties of fluids and mixtures. The known techniques using ultrasound employ methods and apparatus for measuring the attenuation of an ultrasonic wave and correlating such attenuation with the fluid property to be determined. Exemplary of such applications and U.S. Pat. Nos. 3,194,057; 2,966,056; and 2,755,662.
All of the aforesaid techniques and devices have disadvantages regarding accuracy, reproducibility, convenience and/or economies which are overcome by the method and apparatus of the present invention.